Fix modifying taxons in ShopsCaching spec

This was triggering an error via the callback Spree::Product#remove_previous_primary_taxon_from_taxons

spec/features/consumer/caching/shops_caching_spec.rb

@@ -73,7 +73,7 @@ feature "Shops caching", js: true, caching: true do
         expect(page).to have_content taxon.name
         expect(page).to have_content property.presentation
 
-        product.update_attribute(:taxons, [taxon2])
+        product.taxons << taxon2
         product.update_attribute(:primary_taxon, taxon2)
         product.update_attribute(:properties, [property2])